Tomodensimeters according to the state of the art employ conventional X-ray tubes with fixed or rotating anodes which generally comprise a linear cathode surrounded by an electron concentrator and producing an electron beam of rectangular section parallel to the axis of the anode. Generally, the anode surface is beveled or frustoconical so that its generatrices are inclined, on the one hand, relative to the electron beam bombarding it and, on the other hand, relative to the useful beam of X-rays obtained by collimation (by means of a diaphragm) of the radiation emitted by the bombarded surface portion, termed the focus or target area. It has been found that the distribution of the energy of the emitted radiation as a function of its angle relative to the surface normal at the target area is not uniform and that in the anode-cathode plane, defined by the axes of the anode and the cathodic filament, this distribution of radiated energy varies considerably, with a maximum of emission in the direction of the aforementioned surface normal.
Another drawback of the use of these beveled anodes for producing a wide-angle fan-shaped radial beam for irradiating a row of detectors, is that the projection of the real focus or target area on the rectangular face of each of these detectors i.e. the virtual focus, undergoes a distortion which increases with the mean angular deviation from the surface normal so that the detectors located at the ends of the row see only a small part of the virtual focus and consequently receive only a small part of the radiated energy.
In order to compensate for these defects, certain electronic means have been developed. In the present state of the art, X-ray tubes are also employed which comprise a rotating anode whose frustoconical or beveled surface is bombarded by an electron beam of elongated section (practically filiform), oriented radially relative to the axis of rotation of the anode and forming on the frustoconical surface of the latter an elongate thermal focus coinciding with a generatrix of the conical surface. The radiation emitted by this focus is collimated in such manner as to select the rays emitted about the tangent to the frustoconical surface in the region of the thermal focus so as to obtain a fan-shaped radiation with an energy distribution which is more uniform than with conventional substantially pin-point sources employing the same type of tube. As this uniformity is still insufficient, owing to the variation of the angle at which the rays emerge, the use of a wedge-shaped attentuator for compensating for this defect has been proposed.
Further, in these X-ray tubes, when the anode is bombarded by the electron beam, a certain number of secondary electrons are emitted from the thermal focus and are reaccelerated in the anode-cathode space, being thus liable to bombard the anode at points outside the target area and to produce an X-ray radiation, termed an extra-focal radiation, which has an adverse effect on the quality of the desired flat fan-shaped X-ray beam.